Electron beam tube magnetic focusing device



p 1952 TADAKUNI FUJII 3,054,015

ELECTRON BEAM TUBE MAGNETIC FOCUSING DEVICE Filed Sept. 26, 1960 POM/6AG6NERA TOR r/M uva S/G/VA L GENERAT PULSE GNRATOR INVENTOR.

TADA/(UN/ FUd/l BY 42% 1W Aqe nt United States Patent 3,054,015 ELECTRONBEAM TUBE MAGNETIC FGCUSING DEVICE Tadakuni Fujii, Tokyo, Japan,assignor to Nippon Electric Company, Limited, Tokyo, Japan, acorporation of Japan Filed Sept. 26, 1960, Ser. No. 58,551 Claimspriority, application Japan Oct. 3, 1959 1 Claim. (Cl. SIS-3.5)

This invention relates to electron beam focusing and more particularlyto such focusing in a traveling wave tube.

In a traveling wave tube the electromagnetic wave propagates along aninteraction circuit where there is projected an electron beam in fieldcoupling relationship. Because of the relatively long length of theelectron path and because of the space charge forces acting in anelectron beam, it is generally necessary to provide focusing means tokeep the electron flow cylindrical during its traveling past theinteraction circuit. In the past such focusing has generally beenprovided by establishing a longitudinal steady magnetic field along thebeam path. However, in practice the strong magnetic field and highfluxes required in the large gap for such magnetic focusing havenecessitated in the use of larg permanent magnets or solenoids whichhave added much to the bulk and weight of traveling wave tubes.

Through the use of periodically varying magnetic fields it has beenfound that the bulk of magnetic apparatus could be considerably reduced.This is due primarily to better utilization of the magnetic fields. Byarranging a plurality of small permanent magnets periodically in theaxial direction of the electron beam a focusing arrangement is created.This type of construction, however, still contains disadvantages fromthe point of view of size and weight of the tube.

An object of the invention is to provide a simplified and more efiicienttraveling wave tube.

A further object of the invention is to provide in a traveling wave tubean electron beam focusing device which is light in Weight and requiresAC. power for its operation.

A feature of the present invention is to produce a magnetic field forfocusing the electron beam by utilizing alternating current to flowthrough an air-cored solenoid, instead of direct current through aconventional ironcored solenoid of a traveling wave tube, for thepurpose of allowing the tube to operate satisfactorily under pulsedconditions.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawing, wherein The single FIGURE illustrates a travelingwave tube which has a magnetic focusing system in accordance with thepresent invention.

In the figure, I denotes the traveling wave tube and 2 the timing-signalgenerator. An AC. power generator 4- produces an output voltage which issynchronized with the pulses produced by the pulse generator 3 whoseoutput frequency is f. The focusing solenoid 5 and capacitor ice 6connected in parallel across said solenoid 5 is used to form a resonantcircuit whose resonant frequency is equal to the frequency nf (where nis a positive integer), the output frequency of the AC. generator. Theinput and output waveguides 7 and 8 of the said traveling wave tube 1are coupled at both ends of the propagating structure.

The electrons emitted from the electron gun 9 are accelerated by meansof the output puls voltage, shown in the figure by a downwardly directedrectangular pulse, developed by the pulse generator 3 to form anelectron beam. This stream travels along the axis of the tube and isfinally collected at electrode 11 where it is dissipated as heat. Spacedclosely around the beam is a helix 10 capable of propagating a slowwave. The helix is proportioned so that the phase velocity of the waveis small with respect to the velocity of light and is approximately thesame as the velocity of the electron beam; whereby the two may mutuallyinteract along the length of the tube. The focusing magnetic field isproduced by the air-cored solenoid 5 which is of conventionalconstruction. A resonant circuit comprising solenoid 5 and capacitor 6is resonant at the output frequency of the said AC. power generator 4.The current flowing in solenoid 5 is then synchronized with the pulsedelectron beam accelerating voltage. Both the pulse generator 3 and theAC. power generator 4 are excited by the timing signal generator 2.Thus, the ratio of the output frequencies of the two generators 3 and 4are always kept constant. The phase relationship between the two outputsis such that the intensity of the magnetic field produced by the saidsolenoid 5 and the output voltage of the said pulse generator 3 are bothmaximum simultaneously.

A common practice with conventional traveling wave tubes utilizingpulsed operation is that the electron-beam accelerating voltage ispulsed and a constant magnetic field is provided at all times. Keepingthe magnetic field energized during the non-operating periods of time isevidently a waste of power. Such a method had to be resorted toheretofore on account of the difficulty in abruptly changing the amountof current flowing through the coil. This is due to the inductance ofthe solenoid being very large. Thus, the magnetic energy stored in themagnetic field is prohibitively large.

According to the present invention, however, the magnetic energy storedin the coil can be converted into electrostatic energy by being storedin the capacitors during the non-operating period by means of arrangingthe focusing solenoid coil to form a part of a resonance circuit.Therefore, alternating current, instead of direct current will flowthrough the focusing coil. Furthermore, in the present invention, anair-cored solenoid can be used instead of the more conventionaliron-cored solenoid provided the frequency of the current through thesolenoid is selected high enough.

Bulkiness and heavy weight have been considered among the defects ofconventional traveling wave tubes even in the case of a tube with aperiodic-focusing magnet and, consequently, the traveling wave tube ofthe present invention deserves a great deal of merit in view of itssmall size and light weight.

While I have described above the principles of my by way of example andnot as a limitation to the scope of my invention as set forth in theobjects thereof and in the accompanying claim.

What is claimed is:

In a travelling wave tube having an electron emitting cathode, means foraccelerating said electrons, a radio frequency wave propagatingstructure, means for focusing the electrons emitted from said cathodecomprising a solenoid disposed coaxially of said structure, a capacitorshunting said solenoid and forming therewith a resonant 10 2,567,624

circuit, a single phase generator connected across said solenoid forenergizing said resonant circuit at the resonant frequency, pulse powermeans connected to said accelerating means, and synchronizing meansconnected to said power means and said generator for insuring a phaserelationship of maximum pulse power at maximum A.C.

References Cited in the file of this patent UNITED STATES PATENTS Re.21,739 Llewellyn Mar. 4, 1941 Thomson et a1 Sept. 11, 1951 2,902,622 ItoSept. 1, 1959

